Hydrogenation of unsaturated compounds has been investigated in many aspects, particularly with respect to the catalysts employed. For example, U.S. Pat. No. 3,130,237, suggests hydrogenating unsaturated compounds by the use of certain cobalt complexes. U.S. Pat. No. 3,205,218 teaches hydrogenation of olefinic compounds utilizing a complex of a nickel or cobalt compound with certain aluminum reactants. U.S. Pat. No. 3,113,986 is related and suggests hydrogenation in the presence of the reaction products of certain metal alkoxides with aluminum trialkyls. Finally, U.S. Pat. No. 3,595,942 teaches selective hydrogenation of block copolymers with reaction products of aluminum trialkyls and metal alkoxides or carboxylates. In all of these catalyst systems, a ratio of organometallic reducing agent to carboxylate can be found which provides optimum hydrogenation activity. Other organometallic reducing agents also can be used, such as alkyl lithiums.
While the hydrogenation processes taught by the above patents are useful for reducing unsaturation, no means is provided for interrupting the hydrogenation with the capability of resuming hydrogenation if the latter is desired. Several important reasons exist for the desirability of a controlled interruption of this kind. For example, in a number of instances, it is desired to hydrogenate a stated percentage of the double bonds of the compound being hydrogenated. However, there is no direct means for ascertaining during the hydrogenation the extent of reduction which has been achieved. Consequently, it is necessary to rely upon experience for stopping the hydrogenation and recovering the product. This, of course, is an undesirable procedure especially where a given degree or level of hydrogenation may be critical to the quality or eventual end use of the reduced product. On the other hand, if the hydrogenation is stopped such as by the addition of water or the like or introduction of air, then it is virtually impossible to renew hydrogenation without injection of a completely new quantity of hydrogenation catalyst. This is undesirable. Another situation in which interruption is desirable is in the hydrogenation of unsaturated copolymeric compounds wherein it is desirable to reduce one type of double bond without materially affecting a second type of double bond in the same molecule. Thus, it is highly desirable to achieve a process which may be interrupted at will, permit removal of samples of the reaction mixture for testing of residual unsaturation or any other desirable property and at the same time to be able to resume hydrogenation if this is subsequently desirable.